1. Field of the Invention
The invention relates to the field of the propulsion of aircraft by ejection of a gas flow, and relates to a thrust orienting nozzle.
2. Description of Related Art
For aircraft propelled by turbojets, with or without a pilot and for drones, having military applications, one objective is stealth.
Stealth is defined in particular in relation to two parameters: the Radar Cross-Section (RCS) and the Infra-Red Signature (IRS). The RCS is the cross-section likely to appear on a radar, taking account of the geometry of the aircraft. The IRS is the heat signature which the aircraft leaves, in particular from its ejection nozzles.
In order to reduce the RCS, it is preferable for the aircraft not to have any stabilizer or vertical tail unit at the rear of the fuselage. The problem of guiding the aircraft then arises, in particular with regard to its changes of direction. In this case it is proposed, in order to control the aircraft in yaw, to carry out a vectoring, that is to say acting on the orientation of the thrust vector.
To pilot the aircraft by acting on the orientation of the gas jet coming from the nozzle is already known. There are solutions which use mechanical or fluidic means for deflection or orientation of the jet. The control of the gas jet by fluidic injection in the divergent portion of the nozzle is an advantageous solution for the application envisaged above because it harmonizes vectoring and discretion aspects. Many studies have already been carried out on the subject.
In the context of the control of missiles in particular, the fluidic injection technique is already used in the divergent portion of the nozzle. The principle consists in creating an obstruction in the divergent portion of the nozzle by an injection of gas. The deflection of the thrust vector is then produced by the deflection of the flow on traversing the oblique shock wave induced by the obstruction and by the high pressure generated by the detachment of the boundary layer in the vicinity of the injection. This solution has the advantage of not having moving parts, unlike mechanical vector control nozzles. It does however suffer from the following disadvantages:
a large drawing off of engine air (of the order of 5%) is necessary in order to achieve thrust deflections of 15 to 20°;
non-negligible thrust losses are observed because of the engine drawing off and because of the losses when traversing the shock wave;
it has a risk of loss of deflection efficiency in the case of impact of the shock wave on the opposite wall.
According to another known technique, a deformation of the sonic line is carried out. The principle consists in obtaining the deflection of the thrust vector by modifying the shape of the sonic line at the throat of the nozzle. This modification is obtained by two simultaneous injections: at the throat on one wall and in the divergent portion of the opposite wall in a zone close to the throat section. This solution has the advantage of avoiding the formation of a shock wave inducing thrust losses. However, the injection at the geometric throat induces an aerodynamic modification of the throat and therefore has an effect on the output and performance of the engine. In particular, the compressor pumping margin is reduced. Moreover, the effectiveness of the device for controlling the yaw is still to be demonstrated.
Moreover, it is known to control the effective section of the throat of the nozzle by fluidic injection at the throat. The effectiveness of such a device has been proven experimentally and by calculation. It is thus possible to achieve effective cross-section restrictions of the order of 10% with an engine drawing-off of the order of 3%.
In the case of a nozzle such as intended to equip a military drone, an objective of IRS and RCS discretion is coupled with the vector thrust requirement. This leads to designing very flat two-dimensional nozzles, with an elongation of the order of 5 for the IRS and RCS discretions and having a pointed external shape for the RCS discretion. The techniques described above have proven their effectiveness with regard to deflection of the thrust vector in order to compensate for the lack of a vertical stabilizer. However, when they are put into practice for nozzles adapted for this application the following difficulties are observed:
Piloting by fluidic injections in a divergent portion requires a large application surface in order to be effective. This is the case for axisymmetric or two-dimensional nozzles with a slight elongation but not for the nozzles in the envisaged applications. Thus, in the configurations already tested, it appears that the lateral surfaces of the nozzles are rather short and of low height. This greatly limits the effectiveness of a parietal injection.
Injection in the vicinity of the nozzle throat gives rise to a substantial reduction in the flow coefficient by an effect of obstruction of the throat section. This reduction of the throat section has, as already reported above, a big effect on the functioning of the gas generator with, in particular, a reduction of the compressor pumping margin.